High-index contrast waveguide coupler

ABSTRACT

An optical coupler comprises an optical tip having a small cross section. The coupler is tapered from a waveguide width to the small cross section tip. In one embodiment, the length of the taper is approximately 40 um long. The waveguide width is approximately 450 nm wide, and the tip is approximately 150×250 nm wide. The tip couples to an optical fiber, which in one embodiment is approximately 4 um wide. In a further embodiment, the waveguide tapers into a combination of multiple tips to provide a better overlap between a mode profile of the fiber and the coupler.

REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority under 35 U.S.C.119(e) to U.S. Provisional Patent Application Serial Nos. 60/350,294,filed Jan. 17, 2002, 60/375,959, filed Apr. 25, 2002, 60/382,686, filedMay 22, 2002, and 60/382676, filed May 22, 2002, all of which areincorporated herein by reference.

[0002] This application is also related to co-pending U.S. patentapplication (1153.059US1) entitled “High-Index Contrast DistributedBragg Reflector,” and filed on the same date herewith, all of which isincorporated herein by reference.

FIELD OF THE INVENTION

[0003] The present invention relates to waveguide couplers, and inparticular to a tapered high-index contrast waveguide coupler.

BACKGROUND OF THE INVENTION

[0004] An optical fiber is usually formed of a core of dielectricmaterial have an index of refraction slightly higher than that ofcladding surrounding the core. Typical optical fibers are forms of Sio₂.Integrated optical circuit optical waveguides are usually smaller,consisting of a dielectric film. They are useful for interfacing withsub-micron size nanophotonic structures. Optical waveguides have beendifficult to connect to fibers due to index and mode mismatch betweenthe fiber and waveguide. Prior attempts at coupling involved longstructures that are difficult to fabricate, or convert only the modesize.

SUMMARY OF THE INVENTION

[0005] An optical coupler comprises an optical tip having a small crosssection. The coupler is tapered from a waveguide width to the smallcross section tip. In one embodiment, the length of the taper isapproximately 40 um long. The waveguide width is approximately 450 nmwide, and the tip is approximately 150×250 nm wide. The tip couples toan optical fiber, which in one embodiment is approximately 4 um wide.

[0006] In a further embodiment, the waveguide tapers into a combinationof multiple tips to provide a better overlap between a mode profile ofthe fiber and the coupler.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a cross section of an optical coupler in accordance withthe present invention.

[0008]FIG. 2 is a cross section of a tip of the optical coupler of FIG.1.

[0009]FIG. 3 is a cross section of tips of a multi-tip optical coupler.

[0010]FIG. 4 is a perspective view of a single and multiple tip opticalcouplers.

[0011]FIG. 5 is a cross section representation of an xy mode fieldprofile of a single tip optical coupler.

[0012]FIG. 6 is a graph of mode mismatch losses dependent on tip widthfor the coupler of FIG. 5.

[0013]FIG. 7 is a graph of mode mismatch losses due to misalignmentbetween the tip and an optical fiber.

[0014]FIG. 8 is a graph showing mismatch loss dependence on tip widthfor double-tip optical couplers.

[0015]FIG. 9 is a perspective view of a thin waveguide coupled segmentedvertical waveguide

[0016]FIG. 10 is a perspective view of one example of a planar opticalwaveguide having thin waveguide coupled waveguide segments.

[0017]FIG. 11 is a perspective view of a further example of a planaroptical waveguide having thin waveguide coupled waveguide segments.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The following description and the drawings illustrate specificembodiments of the invention sufficiently to enable those skilled in theart to practice it. Other embodiments may incorporate structural, andother changes. Examples merely typify possible variations. Individualcomponents and functions are optional unless explicitly required, andthe sequence of operations may vary. Portions and features of someembodiments may be included in or substituted for those of others. Thescope of the invention encompasses the full ambit of the claims and allavailable equivalents. The following description is, therefore, not tobe taken in a limited sense, and the scope of the present invention isdefined by the appended claims.

[0019] A waveguide incorporating an optical coupler is shown at 100 inFIG. 1, including an xz cross-section of electric field profile at y=0.The waveguide comprises a waveguide section 115, a tapered section 120,and a tip 125 having an end 130 coupled to an optical fiber 135. In oneembodiment, the waveguide 115 is a high-index contrast waveguide. Thetapered section 120 is based on high-index contrast materials, such asSi and SiO₂ or other III-V semiconductor materials having opticalwaveguide properties.

[0020] In one embodiment, the tip has a cross section of less than 250nm for Si—SiO₂, and approximately 150 nm in one embodiment. The tipcouples directly to the optical fiber 135, which has a diameter ofapproximately 4 um in one embodiment. The tip may be positioned directlyadjacent the fiber, or may be adhered to the fiber in one of many knownways such that the tip is approximately centered on an end of the fiber.If not precisely centered, efficiency may decrease, but some amount oflight coupling still occurs.

[0021] The tapered section 120 has a length of approximately 40 um inone embodiment, increasing in width from the tip to the waveguide toapproximately 450 nm. The taper provides matching of the mode of the tipto the waveguide mode. With the tapered section 120 at approximately 40um, both the mode profile and the effective index were converted. Forthe Si—SiO₂, waveguide, the size of the waveguide is approximately450×250 nm. The waveguiding material is Si, and the cladding is SiO₂.Other high-index contrast materials may also be used, such as (any pairof materials that have a high index contrast—Si and SiO2 well knowncharacteristics and easy to process.

[0022] The principle of operation of the coupler relies on alow-confined approach. Using a very narrow optical tip, much smallerthan the wavelength of light, the field in the cross-sectionperpendicular to the propagation in the xy plane, is predominantlyevanescent. In other words, most of the optical power is spread in thecladding region, instead of in the core. This induces a very large modeprofile, of the order of an optical fiber mode. The structuresefficiently couple light with fibers with very low losses, even thoughthe mode profile at the tip is not Gaussian, but mostly exponentiallydecaying.

[0023] Si waveguides have been shown to transmit light at a wavelength,λ, of approximately 1.5 um with very low losses. Using high-contrastmaterials allows miniaturization of the coupler, and presents a lowindex mismatch with a fiber, since most of the field resides in the SiO2region of the tip 130, causing the effective index to be close to thatof the SiO2. Reflections are also negligible.

[0024] Many fabrications methods are available to form the structure ofFIG. 1. Some processes include reactive ion etching, reactive ion beametching, N₂ iron milling and others. Certain photolithography methodsmay also be used to form the taper and tip. Given the submicronstructures, electron beam lithography offers high spatial resolutions.Tapers may also be formed by epitaxial growth.

[0025] The coupling efficiency for the structure of FIG. 1 is believedto correspond to about 60%. This is approximately equal to the overlapbetween the fiber and the tip mode profile (66%). The small discrepancyarises due to losses in the taper. FIG. 2 is a cross section of the modeprofile of the tip 130. The dimensions of the tip are approximately100×150 nm, and the fiber core 135 has a diameter of approximately 4 um.The dimensions may be varied significantly to accommodate differentdesired performances, and different wavelengths of radiation.

[0026] In order to further increase efficiency, a combination ofmultiple tips is used as seen in FIG. 3. A better overlap between themode profile of the fiber and the coupler is achieved. The mode profileof a two tip device is shown in FIG. 3. A first tip 310 is positionedfrom a second tip 320 to create an overlap of the mode profile of thetwo tips with the fiber mode profile of approximately 92%. The distancebetween the two tips adjacent the fiber 330 is significantly less thanthe diameter of the optical fiber.

[0027] Further, in one embodiment, each tip has a height greater than awidth, and the heights of the two fibers are substantially parallel andequally spaced from the longitudinal center of the fiber or waveguide.The two tips create a substantially circular mode profile at theintersection with the fiber or waveguide. In the embodiment shown, thetwo tips are positioned such that the circular mode profile issubstantially concentric with the longitudinal axis of the fiber.Embodiments with more than two tips are also within the scope of theinvention.

[0028] In one embodiment, the waveguide is coupled to optical circuitryformed on an integrated circuit, such as a substrate on which thewaveguide is formed. The waveguide tapers as described, and couplesdirectly to an optical fiber for carrying light to other integratedcircuits on different substrates, or for longer transmission as desired.

[0029] A perspective view of a single taper and double taper waveguidecoupler 405 and 410 respectively are shown in FIG. 4. The basic buildingblock of the structures is based on a short taper 415 withnanometer-size tip 420 composed of the same high-index contrast materialthat composes the waveguide 405. The refractive index of the claddingmaterial 425 and 428 is chosen to be very close to the effective indexof the optical fiber mode. The tip 420 having a width w_(s) is followedby the taper 415 of length l_(s) to the width of the waveguide w_(w).The height h of the waveguide, tip and taper is constant throughout thecoupler in one embodiment. The coupler comprises a planar opticalcircuit. The double taper waveguide 410 has two tips 430 and 435, eachone with width w_(d) separated by s_(d). In one embodiment, the tapersjoin at the waveguide at a greater width, s_(d)+w_(d), and then taperback to the width of the waveguide for a taper length of l_(d2). In someembodiments, the tapers are linear, and in others they follow a curvedform, such as parabolic.

[0030] The tapers can present any geometry (linear, quadratic, cubic,exponential, etc.), but usually the linear is not the most efficientone. One method of manufacturing the couplers utilizes purchasedSilicon-On-Insulator (SOI) wafers, followed by patterning of the Sistructures and then deposit of SiO2. Any other combination of materialsthat can lead to high index contrast, like Si (n=3.48) and SiO2(n=1.46), and are appropriate to micro/nanofabrication can be used.

[0031] The coupler is believed to operate based on the fact that thefield in the narrow optical tip is predominantly evanescent. Thisinduces a very large mode field, of the order of the optical fiber modefield diameter (MFD). This delocalization of the mode field profileincreases the overlap with the optical fiber mode. In addition, most ofthe mode field resides in the cladding region at the tip, causing theeffective index to be close to that of the fiber, which makesback-reflections become negligible. The taper converts both the modeprofile and the effective index in a typical range of tens of microns.In general, a rectangular tip cross-section will present a polarizationdependent behavior, what can be suppressed by introducing a square tipcross-section. In one embodiment, the waveguide height is equal to thetip to result in a square tip cross-section. In a further embodiment, avertical taper is provided from the waveguide height along the taper tothe tip.

[0032] A square cross-section for achieving good power overlap withoptical fiber is about 150 nm×150 nm in one embodiment. With no verticaltaper, the final waveguide will be somewhat about 450 nm×150. In afurther embodiment, the final waveguide has a cross-section 450 nm×250nm, providing effective indexes of 2.51 (TE) and 2.05 (TM). Usingmultiple tips, the coupling efficiency is increased by improving themode overlap of the coupler with the mode of the optical fiber.

[0033] Simulations were performed at λ=1.55 μm for the TE-likepolarization. The single-mode optical fiber used as an input modereference has a circular core of diameter d=4 μm, with indicesn_(core)=1.48, n_(clad)=1.46, which gives a MFD of d_(MFD)=4.9 μm and aneffective index of 1.468. This corresponds to the MFD of a typicalErbium-doped optical fiber. Simulations were performed based on beampropagation method (BPM) and finite-difference time-domain (FDTD)method. The waveguiding and cladding materials are Si (n_(Si)=3.48) andSiO₂ (n_(Si)=1.46), respectively. The applicability of this approach isnot limited only to these materials. The waveguide height and width aretaken as h=250 nm and w_(w)=450 nm, respectively, in order to achievesingle-mode operation.

[0034]FIG. 5 shows a xy mode field profile of the single-tip structurein FIG. 4, where w_(w)=120 nm (the width of the tip) at the tip facet,together with the fiber MFD and tip dimensions. The field is calculatedusing mode solvers based on semi-vector 3D-BPM. The mode field profileat the tip qualitatively matches the fiber MFD. The power overlap andthus the mode mismatch losses depend on the tip width as shown in FIG.5. The maximum power overlap between the optical fiber and tip modes wasabout 94%, obtained for w_(s)=120 nm. This corresponds to mode mismatchlosses of 0.26 dB. In FIG. 6 it is seen that a margin of error of 28 nmin the fabrication process of the tip width can be tolerated for modemismatch losses increase of only 0.5 dB with respect to the optimumperformance. For all values of w_(s), the effective index of thesingle-tip mode is below 1.48, leading to back-reflection losses at thefacet better than 48 dB. FIG. 7 illustrates the mode mismatch losses dueto misalignment between the single-tip structure and the optical fiber,along the x (solid line) and y (dashed line) directions. In order tostay within 1 dB of the minimum mode mismatch losses, a relatively largemisalignment tolerance of 1.2 μm, between fiber and tip, is allowed inboth x and y directions (FIG. 7). In order to convert the mode at thetip facet into the waveguide mode, a tapered transition of lengthl_(s)=30 μm is used by gradually varying both sidewalls in a symmetricparabolic transition towards the final waveguide width, where theparabola vertex is located at the tip. A qualitative picture of thepropagating field, obtained using semi-vector 3D-BPM, is shown in FIG.8. One can see the strong field in the waveguide following the coupler,where some higher order waveguide modes are still present. Using 2D-FDTDlosses were quantitatively assessed. This approach provides a goodapproximation of the losses of the 3D counterpart, since tapering isonly done on the width (xz plane), whereas the height is kept constant.In one embodiment, the taper can effectively convert both mode size andeffective index. In further embodiments, longer tapers and/or differenttaper transitions are used to minimize losses.

[0035] A double-tip coupler, as shown in FIG. 4 is used to furtherimprove power overlap. In this structure one has a larger flexibility inshaping the mode field profile by adjusting both parameters s_(d) thewidth between tips 430 and 435, and w_(d), the width of the tips 430 and435. The xy mode field profile at the double-tip facet is shown in FIG.6 for s_(d)=1.1 μm and w_(d)=105 nm. The maximum power overlap betweenthe optical fiber and tip modes was about 96.4% (mode mismatch losses of0.16 dB), obtained for for s_(d)=1.1 μm and w_(d)=105 nm. FIG. 7 showsthe dependence of the mode mismatch losses on a few geometricparameters. The tolerance to the tip width is increased with s_(d). Forall values of s_(d) and w_(d), the effective index of the double-tipmode is below 1.47, leading to back-reflection losses at the facetbetter than 51 dB.

[0036]FIG. 8 shows a plot of simulated mode mismatch losses versus tipwidth for the two tip taper of FIG. 4. Three spacings of the tips areshown for varying tip width. The tolerance to the tip width is increasedwith s_(d). For all values s_(d) and w_(d), the effective index of thedouple-tip mode is below 1.47, leading to back-reflection losses at thefacet better than 51 dB.

[0037] A distributed Bragg reflector is shown generally at 110 in FIG.9. A waveguide having a high index of refraction has a first end 915,and several sections of approximately equal diameter 920, 925, and 930positioned between the first end 915 of the waveguide and a second endof the waveguide 940. Section 920 is coupled to the first end 915 by athin waveguide section 945. Section 920 and 925 are coupled by a thinwaveguide 950. Section 925 and section 930 are coupled by a thinwaveguide 950, and section 930 and second end 940 are coupled by a thinwaveguide 960. Each thin waveguide has a high index of refraction. Thewaveguide structure, including the ends of the waveguide, the sections,and thin waveguides are surrounded by medium 970 having a low index ofrefraction.

[0038] In one embodiment, the medium is air, having a index ofrefraction of 1. The waveguide structure is formed of silicon, and hasan index of refraction of approximately 3.48. With one set ofgeometries, the effective indices of refraction are calculated at 3.27and 1.45 in the high and low index regions respectively. Losses varywith the width of the thin waveguide, as does the reflectivity.Optimizing the width of the thin waveguide provides high reflectivityand low losses for a distributed Bragg reflector.

[0039] In one embodiment a thin waveguide section in combination withthe medium having a low index of refraction form an optical coupler thatpropagates light. The medium having a low index of refraction isreferred to as a thick elongate cladding portion. The thin waveguidesection is referred to as a thin elongate material that is disposedwithin the cladding. The thin elongate material has a thickness smallerthan the wavelength of the light to be propagated. In one embodiment,the optical coupler has a mode profile that is comparable in size to amode profile of an optical fiber. The thin elongate material is formedof Si and the cladding is formed of SiO₂. The thin elongate material hasa sub micron cross section in one embodiment.

[0040] In a further embodiment, the optical coupler further comprisesadditional thin elongate materials disposed within the cladding having ahigh index of refraction, wherein the additional thin elongate materialhas a thickness substantially smaller than the wavelength of the lightto be propagated. In one embodiment, the additional thin elongatematerials are axially aligned with an axis of the cladding.

[0041] The thin elongate materials are separated from each other tominimize interference from light propagated by them.

[0042] In FIGS. 10 and 11, planar optical waveguides forming distributedBragg reflectors are shown at 1010 and 1110. Reflector 1010 compriseswaveguide sections 1015, 1020, 1030, 1040 and 1050 coupled by thinwaveguides 1055, 1060, 1070 and 1080. In this case, the wires aresubstantially centered on each waveguide section, and have much smallerwidths and heights than the waveguide sections. They are essentiallyfloating between the sections at the geometric center of the crosssection of such sections.

[0043] In FIG. 11, reflector 1110 comprises waveguide sections 1115,1120, 1125, 1130, and 1135 coupled by thin waveguides 1140, 1150, 1160and 1170. In both the waveguides, the indices of refraction for thewaveguide sections and wires are high compared to medium surroundingthem. The thin waveguides in this embodiment are the same height as thewaveguide sections, but are much narrower. They are easily formed usinga single photolithograph step to define both the waveguide sections andthe thin waveguides.

[0044] The planar optical waveguides are formed on a substrate or bufferof a material with lower refractive index. The waveguides are buried ina material with lower refractive index or left with air as the upperlayer. Fabrication is performed using lithographic processes (optical ore-beam) for patterning the device onto the substrate, such as silicon oninsulator substrates, or another appropriate bulk or buffered substrate.Following patterning, reactive ion etching or appropriate depositionprocesses, depending on the type of substrate utilized, are performed tocomplete the device. The devices may be performed in many differentmanners, as the resulting structure and difference in index ofrefraction in surrounding medium are easily obtainable by many differentprocesses, including those yet to be developed.

[0045] The vertical structure of FIG. 9 is formed by epitaxial growth ordeposition using any process, such as MBE, CVD, MOCVD, evaporation andsputtering, as well as any other available process. Materials used areusually Ill-V compounds and alloys, as well as oxides thereof, but othermaterials may also be used. Conventional processes used for fabricatingreflectors in vertical cavity surface emitting lasers are inherentlyappropriate for fabrication of the vertical structures. In a veryconventional process, MBE is used for epitaxially growing or alternateGaAs and AlAs layers, (optical or e-beam) lithography is used forpatterning the cross sectional regions, and plasma or reactive ionetching is employed for transferring the pattern. Using this example,selective etching of AlAs with respect to GaAs layers is performed as afinal step in order to obtain the corresponding narrow AlAs thinwaveguide layers alternated by wide high-index GaAs layers. This leadsto a structure geometrically similar to that of FIG. 9, which presents agenerally cylindrical cross section, although other cross section shapesmay also be used.

[0046] In a further embodiment, an optical type fiber having embeddedone or more thin waveguide structures made with a different index ofrefraction material than the surrounding fiber. The thin waveguidestructures could be located towards the middle of the optical-type fiberor around the periphery. In one embodiment, each thin waveguide carriesa separate optical signal. The thin waveguides are spaced from eachother such that signals carried on the separate thin waveguidestructures would not interfere with each other nor would there be asignificant loss of signal from the thin waveguide. The application ofthis is for telecommunications and perhaps in any optical system wheresimultaneous transmission of multiple separate optical signals aredesired—such as an optical equivalent of multiwire cables used incomputers now (for use in an optical computer). One method of makingsuch fibers comprises arranging high and low index fibers and heatingand drawing them a thin fiber of substantially parallel fibers.

Conclusion

[0047] A new class of easy-to-manufacture, micron-sized devices allowscoupling between an optical fiber and a high-index contrast waveguide,with low losses. The structures are composed of high index contrastmaterials, and consist of one or multiple nanometer-size tips tapered tothe waveguide dimensions. The structures are based on Si/SiO₂ in a 30 μmlong device. The ease of manufacture stems from the fact that onlylateral tapering is necessary, making it widely suitable for few-stepfabrication processes with conventional e-beam lithography. Losses ofthe couplers are governed mainly by the power overlap between the fieldat the tip(s) facet(s) and the fiber mode. One can then envision thelosses to become negligible by increasing this overlap using geometriesthat employ several tips. This class of on-chip devices are very smalland different embodiments are capable of high coupling efficiency,extremely low back-reflections, and ease of manufacture.

1. An optical coupler comprising: an optical tip; a waveguide; and atapered section between the optical tip and the waveguide having acladding of a material having a low index of refraction, and wherein theoptical tip, waveguide and tapered section have a high index ofrefraction.
 2. The optical coupler of claim 1 wherein the waveguide isformed of Si and the cladding is formed of SiO₂.
 3. The optical couplerof claim 1 wherein the tapered section is approximately 40 um in length.4. The optical coupler of claim 1 wherein the width of the optical tipis substantially less than the wavelength of light to be transferredthrough the optical tip.
 5. The optical coupler of claim 1 wherein thetip is coupled to an optical fiber.
 6. The optical coupler of claim 5wherein the optical tip has sub micron cross section, and the opticalfiber has a diameter greater than a micron.
 7. The optical coupler ofclaim 5 wherein the optical tip is approximately 100×250 nm, and theoptical fiber has a diameter of approximately 4 um.
 8. An opticalcoupler comprising: a waveguide having a core with a tapered sectionextending into an optical tip, the waveguide having a cladding about thecore formed of a material having a low index of refraction, and whereinthe core including the tapered section and tip have a high index ofrefraction relative to the cladding.
 9. The optical coupler of claim 8wherein the core is formed of Si, and the cladding is formed of SiO₂.10. The optical coupler of claim 9 wherein the effective index at thetip of the waveguide is close to that of SiO₂.
 11. The optical couplerof claim 8 wherein the cross section of the tip is much smaller than thewavelength of light in the waveguide.
 12. The optical coupler of claim 8wherein optical power is spread in the cladding region proximate theoptical tip.
 13. The optical coupler of claim 8 wherein a mode profileat the tip of the waveguide approximately matches an optical fiber modeto which it may couple.
 14. An optical coupler comprising: a waveguidehaving a core with a tapered section extending into two optical tips,the waveguide having a cladding about the core formed of a materialhaving a low index of refraction, and wherein the core including thetapered section and tips have a high index of refraction relative to thecladding.
 15. The optical coupler of claim 14 wherein the core is formedof Si, and the cladding is formed of SiO₂.
 16. The optical coupler ofclaim 15 wherein the effective index at the tips of the waveguide isclose to that of SiO₂.
 17. The optical coupler of claim 14 wherein thecross section of the tips is much smaller than the wavelength of lightin the waveguide.
 18. The optical coupler of claim 14 wherein opticalpower is spread in the cladding region proximate the optical tips. 19.The optical coupler of claim 14 wherein a mode profile at the tips ofthe waveguide is large, on the order of an optical fiber mode.
 20. Theoptical coupler of claim 19 wherein the modes overlap by approximately92%.
 21. The optical coupler of claim 14 wherein the tips each comprisea height greater than a width, and wherein the two heights are orientedsubstantially parallel to each other, and concentric from each otherwith respect to the longitudinal axis of the waveguide.
 22. The opticalcoupler of claim 21 wherein the two tips create a substantially circularmode profile at the intersection with the waveguide.
 23. An opticalcircuit comprising: a waveguide having a core with a tapered sectionextending into an optical tip, the waveguide having a cladding about thecore formed of a material having a low index of refraction, and whereinthe core including the tapered section and tip have a high index ofrefraction relative to the cladding; and an optical fiber having one endcoupled to the tip at approximately the longitudinal center of theoptical fiber.
 24. The optical circuit of claim 24 wherein the opticalfiber has a diameter of approximately 4 um, the tip has a cross sectionof approximately 150×150 nm, the taper has a length of approximately 40um, and the waveguide has a width of approximately 450 nm.
 25. Anoptical coupler that propagates light comprising: a thick elongatecladding portion having a low index of refraction; and a thin elongatematerial disposed within the cladding having a high index of refraction,wherein the thin elongate material has a thickness smaller than thewavelength of the light to be propagated.
 26. The optical coupler ofclaim 25 wherein the optical coupler has a mode profile that iscomparable in size to a mode profile of an optical fiber.
 27. Theoptical coupler of claim 25 wherein the thin elongate material is formedof Si and the cladding is formed of SiO₂.
 28. The optical coupler ofclaim 25 wherein the thin elongate material has a sub micron crosssection.
 29. The optical coupler of claim 25 and further comprising anadditional thin elongate material disposed within the cladding having ahigh index of refraction, wherein the additional thin elongate materialhas a thickness substantially smaller than the wavelength of the lightto be propagated.
 30. The optical coupler of claim 25 and furthercomprising plural additional thin elongate materials disposed within thecladding having a high index of refraction, wherein the additional thinelongate materials have a thickness substantially smaller than thewavelength of the light to be propagated.
 31. The optical coupler ofclaim 30 wherein the thin elongate materials are separated from eachother to minimize interference from light propagated by them.